1. Field of the Invention
The present invention relates to variable optical attenuator which attenuates a light signal by rotating the polarization of the light signal as the light signal passes through a Faraday element. More specifically, the present invention relates to the angle of rotation of the polarization of the light signal, the structure of an electromagnet and permanent used to rotate the polarization of the light signal, and the control of the power level of the light signal and the power level of light output from the variable optical attenuator.
2. Description of the Related Art
FIG. 28 is a diagram illustrating a conventional optical communication system which uses wavelength division multiplexing. Referring now to FIG. 28, a plurality of optical transmitters (OS1 . . . OSn) 200 transmit light signals at corresponding wavelengths (.lambda.1 . . . .lambda.n). The light signals are generated from a light source (not illustrated), typically a laser diode (LD), within optical transmitters 200. The light signals transmitted by optical transmitters 200 are combined by a multiplexer (MUX) 210 into a wavelength division multiplexed signal which propagates through an optical fiber 220. An optical amplifier 230 amplifies the wavelength division multiplexed signal. A demultiplexer (DEMUX) 240 demultiplexes the wavelength division multiplexed signal into a plurality of individual light signals at wavelengths .lambda.1 . . . .lambda.n. A plurality of optical receivers, or a single tunable optical receiver 250, can be used to detect the individual light signals. An optical frequency controller 260 and an optical frequency standard device 270 can be used to control the transmitting frequencies of optical transmitters 200.
In an optical communications system, it is often required to adjust the intensity (optical power) of light signals. For example, the quality of a signal is determined by the ratio between the intensity of an optical signal and the intensity of noise in the optical signal. This ratio is commonly referred to as the optical signal-to-noise ratio (optical SNR). Therefore, it is often necessary to adjust the intensity of a light signal to increase the optical SNR above a predetermined level.
Moreover, to increase the optical SNR of a wavelength division multiplexed signal in the optical communication system illustrated in FIG. 28, it is usually required for the individual light signals have the same light intensity. However, the level of each light signal undesireably varies according to a variation in the output power of the light source generating the light signal, and according to variations in the insertion loss of optical components in the optical communication system. Also, an optical amplifier typically has a wavelength dependent gain, which thereby causes the various signal lights to have different light intensity.
A variable optical attenuator is typically used to control the intensity of each light signal, and thereby maintain each light signal at the same light intensity. Generally, a variable optical attenuator attenuates, or reduces, the intensity of some of the light signals so that all of the light signals are maintained at the same intensity.
In a conventional optical attenuator, an appropriate substance is attached to a glass substrate so that the light transmissivity varies continuously on the substrate. Attenuation of a light signal is varied by mechanically shifting the position at which the light signal passes through the glass substrate. However, such mechanical shifting of the position of the light signal results in a relatively slow optical attenuator with an undesireably large size. Thus, it is difficult to provide such a mechanical shifting variable attenuator in an optical transmitter.
Japanese Laid-Open Patent Application No. 6-51255 entitled "OPTICAL ATTENUATOR" discloses a variable optical attenuator which does not require a mechanical shifting operation. FIG. 29 discloses such a variable optical attenuator 9. Referring now to FIG. 29, the variable optical attenuator includes a magnetooptical crystal 1, a polarizer 2, a permanent magnet 3 and an electromagnet 4. A light signal is linearly polarized by a polarizer (not illustrated), to thereby provide a linearly polarized light signal 5. Linearly polarized light signal 5 travels through magnetooptical crystal 1 along a light path. Permanent magnet 3 applies a magnetic field which is parallel to the light path. Electromagnet 4 applies a variable magnetic field which is perpendicular to the light path. The variable magnetic field is controllable by controlling the current provided to electromagnet 4. The magnetic field applied by permanent magnet 3 and the magnetic field applied by electromagnet 4 combine together to form a resulting, or composite, magnetic field which rotates the polarization of linear polarized light signal 5 as it travels along the light path through magnetooptical crystal 1. Magnetooptical crystal 1, permanent magnet 3 and electromagnet 4 together form a Faraday rotator 9.
A large optical loss will occur when magnetooptical crystal 1 has a large number of optical domains. However, if the magnetic field applied by permanent magnet 3 is greater than a saturation level, the composite magnetic field becomes greater than the saturation magnetic field. In this case, magnetic domains inside magnetooptical crystal 1 are substantially integrated into one large domain, thereby reducing the amount of optical loss.
As the intensity of the magnetic field produced by electromagnet 4 varies in accordance with the level of current in electromagnet 4, the orientation of the composite magnetic field varies in accordance with the level of the current. The polarization direction of light signal 5 is rotated by the composite magnetic field, in accordance with a physical principle referred to as the "Faraday effect". The degree of rotation (that is, the "Faraday rotation") is related to the intensity of the component (magnetization vector) of the composite magnetic field which is parallel to the light path.
The Faraday rotation .theta. is given by the following Equation (1).
Equation (1): EQU .theta.=V.multidot.L.multidot.H
where V indicates Verdet's constant determined according to the substance forming magnetooptical crystal 1, L indicates an optical path and H indicates a magnetic field intensity.
Referring again to FIG. 29, light signal 5, having its polarization direction rotated, travels to polarizer 2. If the polarization direction of polarizer 2 coincides with the polarization direction of light beam 5, the entire light beam 5 passes through polarizer 2. If the polarization directions do not coincide, only a component of light beam 5 in alignment with the polarization direction of polarizer 2 passes through polarizer 2. If the polarization directions have a 90 degree displacement with respect to each other, light beam 5 does not pass through polarizer 2, thereby providing maximum attenuation of light beam 5. In this manner, the Faraday rotation .theta. can be controlled to determine what portion of light beam 5 passes through polarizer 2.
Japanese Laid-Open Patent Application No. 6-51255 also discloses another type of optical attenuator. Such an optical attenuator is illustrated in FIG. 30. Referring now to FIG. 30, a portion of a light signal supplied by an optical fiber 6a is led to an optical fiber 6b by a birefringent effect provided by birefringent crystals 8a and 8b. Lenses 7a and 7b are used to focus the light signal. A Faraday rotator 9, such as the Faraday rotator 9 illustrated in FIG. 29, is between birefringement crystals 8a and 8b. The proportion of the light signal led to optical fiber 6b with respect to the entire light signal can be controlled by adjusting the Faraday rotation provided by Faraday rotator 9. Thus, the power of the light signal can be variably attenuated.
Whereas the variable optical attenuator illustrated in FIG. 29 requires a light beam to be linearly polarized, the variable optical attenuator illustrated in FIG. 30 does not require a light beam to be polarized in any specific direction.
The variable optical attenuators illustrated in FIGS. 29 and 30 do not require any mechanical shifting operation and, therefore, do not have any moving parts. Therefore, such variable optical attenuators provide improved reliability over conventional variable optical attenuators which require parts to be mechanically shifted.
However, with the variable optical attenuator illustrated in FIG. 29, Faraday element 1 is generally a yttrium-iron-garnet (YIG) plate or a garnet thick film that provides a Faraday effect. Unfortunately, the Faraday rotation provided by such a Faraday element generally has a wavelength dependence and a temperature dependence of the rotation.
FIG. 32 lists the wavelength dependence and the temperature dependence of the Faraday rotation provided by the Faraday element, and variations in the Faraday rotation with respect to a variation in the wavelength or the temperature. Measurements are taken of a Faraday rotator that produces a 45-degree Faraday rotation at 1550 nm. The garnet thick film changes its characteristic when its composition is changed. FIG. 32 shows a relatively large change. Negative signs in FIG. 32 indicate that the Faraday rotation decreases as the wavelength or the temperature increases.
FIG. 31 is a graph illustrating a relationship between the magnetic field strength H and the Faraday rotation. Referring now to FIG. 31, as the magnetic field strength H increases, the Faraday rotation increases with a gradient V.times.L. The Faraday rotation saturates beyond a certain level of the magnetic field strength H. The saturation indicates that the magnetic domains inside the magnetooptical crystal are integrated into one domain. FIG. 31 shows that the gradient V.times.L changes as the temperature or the wavelength changes. As a result, Verdet's constant has an undesirable wavelength dependence and temperature dependence.
Thus, variable optical attenuators as illustrated in FIGS. 29 and 30 are undesireably dependent on wavelength and temperature. In addition, the variable optical attenuator illustrated in FIG. 30 contains a slight polarization dependent loss.